The present invention pertains to a process for preparing aqueous encapsulated pigment dispersions. These dispersions may be used in the preparation of aqueous inkjet inks, pigmented paints, and the like. More particularly, the invention relates to a dispersion process where a dispersed pigment is prepared, monomers are added to the dispersed pigment and polymerization is initiated to obtain an encapsulated pigment.
Dispersion methods are commonly used to disperse particles. A variety of dispersion processes have been described for dispersing particles. For ink jet inks pigments are dispersed with polymeric dispersants. Recently, processes have been described where the polymeric dispersed pigments are subject to a process that results in encapsulated pigments.
U.S. Pat. No. 6,262,152 describes preparing encapsulated pigments where the encapsulation is achieved by using polymeric dispersants which have reactive sites and adding monomers that have substituents that can react with the reactive sites of the polymeric dispersant. For instance, the dispersant has isocyanate reactive groups and an isocyanate is added to react with the polymeric dispersant.
U.S. Pat. No. 7,741,384 describes an encapsulation process where both the pigment dispersion and the monomer dispersion are separately mixed utilizing a surfactant to independently stabilize the dispersion and the monomer.
US20080064786 describes a water insoluble polymeric dispersant for the pigment and two crosslinking steps, the first occurring in a ketone/water solvent and crosslinks the core of the polymer followed by a second crosslinking for the shell of the polymer.
US200700227401 describes an encapsulation process where the monomer is stabilized via a miniemulsion which is stabilized by a hydrophobic organic compound.
U.S. Pat. No. 7,544,418 describes an encapsulation process in which there is a polymer-encapsulated pigment where the polymer has both an acidic monomer and a non-acidic monomer.
There has been effort in the art directed at improving the stability of pigment dispersions. The effort to improve dispersion stability to date has included improvements in the processes used to make the dispersions, the development of new dispersants and the exploration of the interaction between dispersants and pigment particles, and between dispersants and aqueous vehicle. While much of the effort has general application at improving dispersion stability, some of that effort has not found utility in particular applications. For example, the pigment dispersions used in ink jet printing applications have very unique and demanding requirements. It is critical that ink components comprising pigment dispersion remain stable, not only in storage but also over repeated jetting cycles. It is also desirable that the pigment dispersions offer good durability, good rub-fastness, wet-fastness and highlighter pen fastness. As the ink jet industry moves to page-wide array printing the requirements for repeating jetting cycles may be an order of magnitude higher than the traditional Small Office/Home Office market. These emerging needs require improved pigment dispersions.
A need exists for highly stable, higher-quality and different property inks for ink-jet applications. Although improvements in polymeric dispersants have significantly contributed to improved ink-jet inks, the current dispersants still do not provide inks with the requisite stability, durability, optical density and chroma needed for ink jet applications. The present invention satisfies this need by providing encapsulated pigment dispersion based on monomers which have a Lower Critical Solution Temperature and optionally, acrylate and acrylic monomers, which are, in turn, polymerized. This results in an encapsulated pigment that has more stability, better jetting without compromise of color properties.